This application claims the priority of German Patent Application No. P 199 10 438.7, filed Mar. 10, 1999, which is incorporated herein in its entirety by reference.
The invention relates to an infinitely-variable pulley drive having two pulleys, respectively disposed on the drive shaft and the output shaft, with a transmission element that circulates between the pulleys, and pressing forces for setting and maintaining the gear transmission being generated by a pressure medium on the drive and output pulleys. A sensor is disposed on the drive shaft that receives torque and generates the pressing forces as a function of the load. The pressure medium flows to the sensor with the output-side pressure and the sensor generates a torque-dependent pressure through a torque-dependent relative movement between at least two valve parts of a valve, thereby supplying the necessary load-dependent pressing force. The sensor operates as a torque-dependent pressing device having oppositely-located pressing cam tracks and, between the cam tracks roller bodies are inserted. The first ring of the device, which has one half of the pressing cam tracks, is fixedly secured to the shaft to co-rotate therewith. The other half of the pressing cam tracks is disposed on a second ring, which is disposed to be displaced axially on the shaft, between the first ring and a cylinder that is fixed to the shaft. The second ring and the cylinder form a first cylinder-piston aggregate that is acted upon by the output-side pressure medium. The second ring and the shaft form a valve for the return flow of the pressure medium from the first cylinder-piston aggregate via a shaft bore, with which the chamber between the first and second rings is connected. A chamber between the first and second rings is closed by a cylinder jacket that radially adjoins one of the rings on the outside, is oriented toward the other ring and is coaxial to the drive shaft, the jacket forming a second cylinder-piston aggregate with the other ring.
In such pulley drives, the arrangement is typically such that one of the pulleys can be axially displaced on each shaft, but is connected, at least indirectly, to co-rotate with the shaft, and is embodied as a pressure cylinder of a piston fixed to the shaft, to which the pressure medium is metered by a rectangular control plunger for setting and maintaining the gear transmission, the plunger being, for example, connected via a control lever to one of the axially-displaceable pulleys, while the two other pulleys are fixed axially and against relative rotation with respect to the shaft.
A pulley gear of the type mentioned at the outset is known from, for example, German Patent Document No. 28 28 347. This document also discloses a design in which a torque sensor is seated on the drive shaft of the gear and is charged by the pressure-medium pressure present in an output-side cylinder-piston aggregate.
The fact that the chamber between the first and second sensor rings is closed to the outside in the known gear to form a second cylinder-piston aggregate, as described in detail in the cited document, serves to form another chamber outside of that chamber in order to damp the movement of the second, axially-displaceable ring in one of the two directions of movement with the aid of the pressure-medium volume located in this chamber.
An aspect of the nature of pulley gears of the present type is that, with respect to a particular drive torque, the necessary pressing force between the output-side pulleys and the transmission element circulating between them depends on whether the transmission position of the gear is set at slow or fast output ratio settings. At slow output ratios (i.e., a slower rotation speed on the output side than on the input side) a higher pressing force is required than when the gear is set at faster output ratios. The same is true when, for example, in an application in a motor vehicle, the total overall speed of the gear increases, which also requires a corresponding increase in the level of the pressing forces.
With these circumstances in mind, in gears of the present type, in which the pressing force between the pulleys and the transmission element is determined by a torque sensor, it is desirable to set the output-side pressing force at a higher value when the transmission position is set for a slow output ratio, or when the transmission is operating at a higher total overall gear speed. It is also desirable to avoid overly-high pressing forces when the transmission is set for fast output ratio operation or operating with a low total overall gear speed.
It is an object of the invention to provide an infinitely-variable pulley gear that avoids the occurrence of overly-high pressing forces when the transmission is set for fast output ratios or operating at a low total overall speed, that also provides a sufficiently high pressing force for transmission positions with a slow output ratio or operation at a high total gear speed.
This object and others to become apparent as the application progresses, are accomplished by the invention, according to which, briefly stated, the surface which is exposed to the pressure medium and which forms part of the second ring has, at its side having the cam track, a greater area than at its side oriented toward the first cylinder-piston aggregate.
This feature according to the invention utilizes the effect exerted by centrifugal force on the pressure medium located in the second cylinder-piston aggregate. According to this effect, at high rotational speeds the pressure medium is exposed to a correspondingly higher centrifugal force, and exerts a correspondingly higher pressure on the axially displaceable, second ring. If the pressure-charged surface of the second ring on the side facing the second cylinder-piston aggregate is larger than on the side facing the first cylinder-piston aggregate, at high output-side speeds at the second ring an additional control variable results at the second ring, which likewise results in an additional throttling effect of the torque sensor, and thus a correspondingly higher pressing force on the output side than at low speeds, without the gear being exposed to an overly high pressure in transmission positions with fast output ratios. The same applies for the above-discussed total gear speed.
The features of the invention, which can employ simple, automatically-acting means, thus avoid application of too much pressure, so the gear operates extremely cost-effectively and reliably.
A design that has proven useful according to the invention is for the outside or major diameter of the second cylinder-piston aggregate to be larger than that of the first cylinder-piston aggregate, with an identical, common inside or minor diameter.
The subject of the invention can be readily implemented in different torque sensor designs, depending on how the drive-side torque is to be introduced because of further structural requirements. For example, the shaft may be divided into segments between the first and second rings, and the torque may be introduced into the gear by way of the shaft part supporting the first ring. It is also possible, however, to introduce the torque into the gear by way of the second ring, in which case the drive shaft is not segmented. Teeth provided at the outer circumference of the second ring, for example, for transmitting force can permit the introduction of the torque via the second ring. Finally, to provide a space-saving design, it can be advantageous for the cylinder for the first cylinder-piston aggregate to be formed by the adjacent pulley that is supported in an axially fixed manner, and fixed against relative rotation with respect to the drive shaft, so that providing a special component for the cylinder is not necessary.